Stereolithographic rocket motor manufacturing method

ABSTRACT

A hybrid rocket motor is manufactured by photopolymerizing the solid fuel grain in a stereolithography method, wherein fuel grains in a plastic matrix are deposited in layers for building a solid fuel rocket body in three dimensions for improved performance and for a compact design,

REFERENCE TO RELATED APPLICATION

The present application is a continuation of and claims priority under35 U.S.C. §120 to U.S. patent application Ser. No. 12/072,717, filed onFeb. 28, 2008 now U.S. Pat. No. 8,225,507, titled “StereolithographicRocket Motor Manufacturing Method,” which is related to co-pending U.S.patent application Ser. No. 12/072,918, filed on Feb. 28, 2008, andtitled “Radial Flow Stereolithographic Rocket Motor,” and to co-pendingU.S. patent application Ser. No. 12/074,001, filed on Feb. 28, 2008, andtitled “Buried Radial Flow Stereolithographic Rocket Motor.” Each of theforgoing applications are hereby incorporated by reference herein intheir entirety.

FIELD OF THE INVENTION

The invention relates to the field of solid fuel hybrid rocket motors.More particularly, the present invention relates to solid fuel hybridrocket motors having internal radial flows and manufactured usingstereolithography.

BACKGROUND OF THE INVENTION

Hybrid motors have recently been given greater attention in the spacecommunity. Hybrid rocket motors use reactants of different physicalphase states, usually a solid fuel such as rubber and a gaseousoxidizer, such as nitrous oxide. Hybrid motors do not generally deliverthe performance of liquid motors. However, hybrid motors are safer andsimpler to build and to operate. Hybrid motors can have good performancebut often have problems maintaining the proper fuel to oxidizer ratioover the duration of the burn. Hybrid motors disadvantageously also tendto be physically long along the rocket motor axis for the same reasons.Hybrid motors can have complicated systems for introducing the gaseousoxidizer portion at different positions length-wise in the fuel section.

Hybrid solid fuel bodies are generally two-dimensional shapes extrudedinto the third dimension, for a simple example, a thick-walled tubeextruded along the length of the tube. Such a tube is characterized ashaving a center axial flow channel. The oxidizer is injected through anintake opening and into the solid fuel body and out through a nozzle asexhaust. The fuel is ignited by an igniter positioned proximal to wherethe oxidizer first contacts the fuel near the intake. The solid fuelbodies generally have a center elongated flow channel through which theoxidizer flows after ignition for ablating the fuel on the side walls ofthe center elongated flow channel. The fuel is burned on the internalsurface effectively ablating the solid fuel interior walls. As the fuelis burned, the combustion becomes oxidizer rich. Oxidizer rich burningprovides poor burning efficiency of the solid fuel. Complex fuel grainshapes are sometimes used to increase the amount of surface area in theelongated center flow channel, but sometimes at the risk of anunsupported section of fuel breaking off and plugging the nozzle,causing a catastrophic failure of the hybrid motor. As the fuel burnsthrough the elongated center flow channel, the oxidizer burns the insideof the channel. The growing diameter of the elongated center flowchannel changes the ratio between the oxidizer flowing in the channeland the exposed burning fuel on the side walls of the elongated centerflow channel. The hybrid rocket motor suffers from changing oxidizer tofuel ratio. The oxidizer to fuel ratio becomes oxidizer rich and therebywastes available oxidizer that could otherwise be used for more burningof the fuel.

Another problem that is associated with hybrid motors, at least for usein launch vehicles, is low regression rates, typically one third of thatof composite solid propellants. Regression rate is the depth-wise rateat which the fuel is removed from the surface where burning occurs. Thisis a factor in the development of rocket engine thrust. A great amountof research has gone into replacing the solid rocket boosters on theSpace Shuttle with hybrid motors only to show that hybrids suffer fromlow regression rates, which may make replacing large solid motors verydifficult. Increased surface area could alleviate this problem.

Stereolithography is a well-known method of building three-dimensionalshapes. Stereolithography is generally regarded as a rapid prototypingtool and is typically used to create mock-ups or models for checking thefit, function, and aesthetics of a design. Stereolithography is planarlithographic layering process for building of a three-dimensional solid.Stereolithography uses a platform or substrate that is repeatedlyimmersed in a photopolymer bath. The exposed photopolymer surface isprocessed one layer at a time effectively adding many patterned layersupon each other in turn. The light from a moving laser beam exposes andcures the thin two-dimensional layer of photopolymer. With eachsuccessive immersion, a new layer of photopolymer is added and athree-dimensional overall shape is eventually made.

There are several rapid prototyping techniques. Stereolithography uses aphotopolymer and a curing mechanism. Fused deposition and 3D printingmodeling rapid prototyping processes melt plastic and inject the plasticthrough a moving nozzle or lay down a field of granules, which areselectively bonded together with a binding agent or sintered togetherwith a powerful laser heat source. In all cases, a three-dimensionalform is created under computer control by building up two-dimensionallayers.

A relatively small cost is required for added design complexity usingstereolithography because no dedicated physical tooling is required forthis process. For example, a complex buried helical path, for example,can be fabricated, as well as simple straight paths and channels. Manyother desired features can be fabricated in the solid usingstereolithography, including pitted, rutted, or undulating surfaces madefrom a plurality of photopolymer layers. The plastic part fabricatedusing stereolithography have been prototypes unsuitable for space usage.

U.S. Pat. No. 5,367,872, entitled A method and apparatus for enhancingcombustion efficiency of solid fuel hybrid rocket motors, issued Nov.29, 1994 to Lund and Richman, teaches a hybrid rocket motor using aplurality of axially aligned fuel grains having multiple axialperforations. Lund claims a hybrid rocket motor comprising a combustionchamber having aft and forward sections, and a plurality of solid fuelgrains. Each grain contains more than one perforation. The fuel grainsare cartridge loaded into the combustion chamber along a rocket motoraxis such that the perforations of at least two solid fuel grains aremisaligned with the perforations of an adjacent solid fuel grain so thatthe fuel grains are arranged within the combustion chamber to allow gasflow through the perforations in a direction substantially parallel tothe rocket motor axis. The individual cartridges can be rotated within acombusted chamber for aligning the perforations. Lund teaches that theflow direction is substantially parallel to the axial elongated centerflow channel. Lund teaches that fuel grain cartridges, which are fueldisks having perforated apertures, can be aligned in a combustionchamber. Each fuel grain cartridge has a plurality of alignedperforations through which oxidizer can flow to burn the solid fuel. Theperforations are aligned so that the gas flows in all of perforationchannels are parallel to each other, substantially in the same axialdirectly between an intake and exhaust. When all of the flow through thesolid fuel is in the same parallel direction, each of the perforationsexperience like changing of the oxidizer to fuel ratio creating unevenburning.

Existing hybrid rocket motors suffer from oxidizer rich burning, limitedparallel axial flow configuration, limited length of the axial channelburning, collapsing of solid rocket bodies, relatively low regressionrates and limited extruded channel shapes. These and other disadvantagesare solved or reduced using the invention.

SUMMARY OF THE INVENTION

An object of the invention is to provide a hybrid rocket motor madethrough stereolithography.

Another object of the invention is to provide a hybrid rocket motorhaving non-axial gas flow.

Yet another object of the invention is to provide a hybrid rocket motorhaving an undulating channel gas flow.

Still another object of the invention is to provide a hybrid rocketmotor having a radial channel gas flow.

A further object of the invention is to provide a hybrid rocket motorhaving a buried channel gas flow.

Yet a further object of the invention is to provide a hybrid rocketmotor having a non-axial gas flow for altering an oxidizer to fuelratio.

Still a further object of the invention is to provide a hybrid rocketmotor having a non-axial gas flow and a fuel core support for supportingthe fuel body core during the burning of internal side walls of a fuelbody of the hybrid rocket motor.

And another object of the invention is to provide a hybrid rocket motorthat is longitudinally compact with the burning in a radial direction.

And another object of the invention is to increase the effectiveregression rate of a hybrid rocket motor by the incorporation of smallvoids that increase the surface area available for combustion.

The invention is directed to a hybrid motor including a fuel grain thatprovides for radial gas flow. The radial gas flow can be used to controlthe oxidizer to fuel ratio during burning of the rocket fuel. Internalsupports made from the fuel can be used to support during burninginternal solid fuel cores forming radial channels. Exhaust and intakemanifolds and brackets can be used in combination with the internalsupports for securing together the rocket motor during a complete burn.

In a first aspect of the invention, the solid fuel body and solid fuelcore is created by a manufacturing method by writing a three-dimensionalcomputer model into a stereolithography polymer that is combustible. Thepolymer includes rubbers and plastics. In some instances, rubber can beused instead of plastic. The plastic part fabricated bystereolithography can be the actual fuel section flight vehicle for usein space. The cured plastic fuel material can be made to be hard andstrong. The cured plastic fuel can be burned with an oxidizer to producethrust for use as part of a rocket motor. A hybrid rocket motor can bemanufactured by photopolymerizing the solid fuel in a stereolithographyrapid-prototyping type machine. Using stereolithography, fuel grains ofany size and shape can be achieved with improved performance in compactdesigns. Rapid-prototyping stereolithography is used for producing ofrocket motor fuel grains, bodies, and cores which can incorporatefeatures that can provide compact packaging and efficient burning. Afterbuilding of the fuel grain, including the fuel body, fuel core, and fuelsupports, manifolds and brackets can then be used to hold the fuel graintogether during burning.

In a second aspect of the invention, convoluted burn channels of anyshape and length can be formed in the fuel body to allow for greatereffective combustion length than the physical length of the motor, andtherefore more complete oxidizer consumption. Nonaxial channels arereferred to as radial channels. The radial channels have a portion thatis not in parallel alignment with a general axial gas channel flowextending along the length of the rocket fuel body. The radial channelscan be used to define the amount of initial exposed surface area thatchanges as the burn proceeds. The use of radial channels providesgreater control over the burn profile including the amount burned andthe oxidizer to fuel ratio using complex three-dimensional shapes thatalso allow for stronger fuel bodies to be built. In a third aspect ofthe invention, buried channels are formed in the rocket body and core.As the oxidizer burns, the fuel from a channel, the channel side wallsare ablated as fuel is burned. After some amount of burning, a buriedchannel is exposed through which burning starts. The use of buriedchannels allows for the design of different burn profiles as desired.Closed voids, embedded nearly throughout the fuel can increase theoverall regression rate and thrust as the burning surfaces expose them.

The stereolithographic hybrid rocket motors with radial channels can bemade in complex shapes for controlling the combustion profile. Thestereolithographic hybrid rocket motors with radial channels are wellsuited for use as picosatellite thrusters, but can be scaled up tolarger sizes as desired. Theoretically, special stereolithographymachines could be built to fabricate motors of almost any size. Theseand other advantages will become more apparent from the followingdetailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of an undulating radial channel solidfuel stereolithographic rocket motor.

FIG. 2 is a cross section view of a parallel radial channel solid fuelstereolithographic rocket motor.

FIG. 3 is a cross section view of a buried radial channel solid fuelstereolithographic rocket motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto FIG. 1, an undulating radial channel solid fuel rocket motor includesa fuel grain comprising a solid fuel body, a solid fuel core, andoptional core support. Preferably, the fuel grain is completely made ofa photopolymer comprising fuel disposed using stereolithography. Thefuel grain includes an entry having an entry flow and an exit having anexit flow. The entry and exit flows are in axial alignment for axialreference. An intake manifold providing an inlet, includes an intakethat is an aperture through which an oxidizer flows designated as anoxidizer flow. The intake may also include multiple apertures tointroduce a larger volume of oxidizer without exacerbating the oxidizerto fuel ratio problems that might occur by flowing all of the oxidizerfrom a single inlet. An exhaust manifold includes a nozzle through whichexhaust flows designated as an exhaust flow. Preferably, the oxidizerflow and the exhaust flow are respectively in parallel coincident axialalignment with entry flow and exhaust flow. The gas flow in the fuelgrain is constricted to provide multidirectional gas flow within thefuel grain. During stereolithographic manufacture of the fuel grain, thesolid fuel core is disposed to form an undulating radial channel throughwhich is at least a portion of radial flow. The gas flow axially entersthrough the entry and then radial flows around the solid fuel corethrough the exit in the exit flow. The radial flow is broadly defined asbeing non-axial flow in reference to the entry flow or the exit flow. Anigniter is disposed near the intake for igniting the fuel grain alongthe inside interior walls of the entry, through the undulating radialchannel, and to the exit. An ignition contact can be used to route anignition current along an ignition conductor to the igniter. An ignitionmeans may consist of several individual igniters, which may be usedserially to allow several re-lights of the rocket motor. The ignitersare typically disposed near and between the intake and the entry. Asshown, the two igniters are disposed in the intake manifold and abut thesolid fuel body of the fuel grain. A complete system to this hybridrocket motor would be structurally similar to other hybrid motorsystems. For example, a complete hybrid motor system would include anoxidizer tank, not shown. The system would also include a controller anda valve to control the oxidizer flow on command. The controller wouldalso provide ignition signals on the ignition contact on command.

The method of making the stereolithographic rocket motor includes thesteps of repetitively disposing a photopolymeric plastic fuel in layers,exposing each of the layers to photolithograpically curing illuminationand removing unexposed portions. The removing step is for definingwithin the fuel grain, the entry, the exit, the solid fuel body, solidfuel core, and a core support. The removing step also for defining inpart a radial channel extending in part between an entry and an exit, sothat the flow is multidirectional within the fuel grain. The fuel grainbetween the entry and exit is then mechanically secured using opposingintake and exhaust brackets. The intake manifold and exhaust manifoldare coupled to the brackets. Ignition contact, ignition conductor, andigniters are disposed near the intake aperture. The igniters can bedisposed in the fuel grain or in the intake manifold. The igniterspreferably abut the fuel grain for efficient ignition of the fuel grain.

In the case of the undulating radial channel solid fuelstereolithographic motor, the solid fuel core resembles a circular diskwith undulating outer surfaces mating to undulating inner surfaces ofthe solid fuel body. The core must be supported to maintain theundulating channel between the core and the solid fuel body. A coresupport is formed so that the circular disk does not extend a fullcircle but is rather suspended within a solid fuel body by the coresupport. The core can be viewed as a simple cylinder extending throughthe solid fuel core and into the solid fuel body for suspending the coreduring a burn.

Referring to FIGS. 1 and 2, and more particularly to FIG. 2, a parallelradial channel solid fuel stereolithographic rocket motor preferablyincludes a stereolithographically fabricated fuel grain comprising asolid fuel body and a solid fuel core. The fuel grain may include anoptional core support, not shown, for supporting the core within thebody. Preferably, the fuel grain is stereolithography made of aphotopolymer comprising fuel. The fuel grain includes an entry having anentry flow and an exit having an exit flow. As preferably shown, theentry and exit flows are in axial alignment for axial reference. Anintake manifold includes an intake that is a cylindrical aperturethrough which an oxidizer flows as an oxidizer flow. An exhaust manifoldincludes a nozzle through which exhaust flows as an exhaust flow.Preferably, the oxidizer flow and the exhaust flow are respectively inparallel coincident axial alignment with entry flow and exhaust flow.Gas flow is passed through the parallel radial channels within the fuelgrain so as to provide multidirectional gas flow within the fuel grain.In one direction, such as through the entry and exit, gas flows in anaxial direction. In another direction, the gas flows radially outwardfrom the entry and radially inward toward the exit. Duringstereolithographic manufacture of the fuel grain, the solid fuel core isdisposed to form the parallel radial channels through which flows atleast a portion of the radial flow. The gas flow axially enters throughthe entry and then radial flows around the solid fuel core through theexit in the exit flow. The radial flow is broadly defined as beingnon-axial flow in reference to the entry axial flow or the exit axialflow. An igniter is disposed in the intake for igniting the fuel grainalone along the inside interior walls of the entry, and along theundulating radial channel, to the exit. An ignition contact can be usedto route an ignition current along an ignition conductor to the igniter.The igniter may consist of several individual igniters, which may beused serially to allow several re-lights of the rocket motor. Theigniters are typically disposed near and between the intake and theentry. As shown, the two igniters are disposed in the intake manifoldbut do not abut the solid fuel body of the fuel grain. The exhaustbracket is shown with an upwardly extending flange for improved securingof the fuel grain.

Referring to all of the Figures, and more particularly to FIG. 3, aburied radial channel solid fuel stereolithographic rocket motorpreferably includes a stereolithographically fabricated fuel graincomprising a solid fuel body and a solid fuel core. The fuel grain mayinclude an optional core support, not shown, for supporting the corewithin the body. The fuel grain includes an entry having an entry flowand an exit having an exit flow. As preferably shown, the entry flow andexit flow are in axial alignment for axial reference. An intake manifoldincludes an intake that is a cylindrical aperture through which anoxidizer flows as an oxidizer flow. An exhaust manifold includes anozzle through which exhaust flows as an exhaust flow. Preferably, theoxidizer flow and the exhaust flow are respectively in parallelcoincident axial alignment with entry flow and exhaust flow. The gasflow in the fuel grain is constricted by buried channels to providemultidirectional gas flow within the fuel grain after an initial burnperiod. The buried radial channel fuel grain includes an elongatedcenter axial channel extending straight between the entry and the exit.During the initial burn, the side walls of the center channel are burnedaway. At some point in time, the burning of the side walls exposes theburied radial channel, after which, the gas flow diverges in part intothe buried channel between the entry and exit. When gas enters theburied channel, radial flow will start in part through the buriedchannel. As such, the gas flow is initially in only one axial direction,such as through the entry, straight channel portion, and the exit, whereall of the gas flows in the axial direction. In another direction, thegas flows radially outward from the entry and radially inward toward theexit after the buried channel is exposed by the side wall burning of theentry, straight, and exit portions of the main axial channel. Duringstereolithographic manufacture of the fuel grain, a solid fuel may bedisposed to form the radially extending buried channel through whichflows at least a portion of radial flow after the initial burn period.The gas flow axially enters through the entry and then radially flowsaround the solid fuel core through the buried channel in radial flow tothe exit in axial exit flow. The radial flow is broadly defined as beingnon-axial flow in reference to the entry axial flow or the exit axialflow. An igniter is disposed in the fuel grain for igniting the fuelgrain alone along the inside interior walls of the entry, buried radialchannel, to the exit. An ignition contact can be used to route anignition current along an ignition conductor to the igniter. The ignitermay consist of several individual igniters, which may be used seriallyto allow several re-lights of the rocket motor. The igniter can bedisposed radially about the intake flow for maximum ignition. Theigniters are typically disposed near and between the intake and theentry. As shown, the two igniters are disposed in the top of the fuelgrain and abut the intake bracket. The exhaust bracket is shown with anupwardly extending flange for improved securing of the fuel grain.

The fuel grain channels are designed to provide physical structure andplumbing of combustion gases. Many new shapes are possible, for example,the fuel grain might have two main chambers of voids, one surroundingthe other, separated by fuel. Linking these chambers could be an arrayof channels, which can be oriented for the best trade-off between goodoxidizer mixing and good gas flow. The number, size, and shape of theburied channels would be determined by the surface area and themechanical strength of the supporting shapes. An igniter section withmultiple igniters, conduction lines, and ignition contacts would beincorporated into the end of the fuel grain shape and mounting brackets.An igniter circuit card and nozzle retainer can also be included. Oneach end, groves for o-ring seals can be designed for improved sealingof combustion gas confined to the axial and radial burn channels. As maynow be apparent, the stereolithographic rocket motor ignition sectioncould further include sensing and control electronics. A thin filmelectronic circuit can be disposed on the brackets or in the fuel grain,though care is needed so that the combustion of the plastic fuel doesnot destroy the electronic circuit early in a complete burn cycle.

Many variations of the fuel grain are possible. The entry flow and exitflow could have a differential direction of ninety degrees, with theexit flow pointing orthogonal to the entry flow. The fuel grain is madeso long as there are internal gas flows is in a plurality of directionsthrough the fuel grain. For example, a radial channel could be made inparallel alignment to an entry flow, both orthogonal to exit flow. Thefuel grain may further consist of a solid oxidizer mixed with the liquidphotopolymer and cured in the same fashion as the unalteredphotopolymer. This partial load of solid oxidizer would not sustaincombustion, but may allow hybrid rocket motor operation with a smallerfluid oxidizer tank and at a lower tank pressure. When the fuel graincontains sufficient oxidizer, an external oxidizer tank may not beneeded with the motor not even having an intake. Using other rapidprototyping techniques such as laser sintering or 3D printing, smallaluminum particles can be added to the medium to produce a moreenergetic fuel than can be obtained with a polymer alone.

Compact hybrid rocket motors can be made from polymeric fuels usingstereolithography manufacturing methods. A solid or hybrid rocket motorwould benefit from channel structure manipulation to control the burnprofile. Various methods normally used for rapid prototyping can be usedto form rocket propellant grains with complex three dimensionalstructures including internal channel structures.

In Selective Laser Sintering, a field of powder is laid down and a laserselectively melts or sinters the powder particles to form a thincontinuous film. Another powder layer is applied and melted on top ofthe first layer. This is repeated until a 3-D shape is built up.Channels of un-sintered material can be cleared of unincorporatedparticles to produce the flow channels and ports of the rocket motorfuel grain.

In Fused Deposition Modeling, a bead of molten material is extrudedthrough a nozzle like extrusion head. As the head is moved, a trail ofextruded material solidifies behind it. A support material is laid downat the same time so that otherwise unsupported design features can besupported while the shape is built up. The process is repeated withanother layer on top of the first and this is repeated until a 3-Dobject is built up. The support material is removed, usually bydissolving in water, leaving the channels and ports of the rocket motorfuel grain.

In Stereolithography, a film of liquid photopolymer is selectively curedby exposure to light, usually from a laser. After the first layer ofphotopolymer is cured, the cured layer is submerged and another layer iscured on top of it. Uncured regions are left liquid. These uncured areasdefine channels and ports of the rocket motor fuel grain as the liquidphotopolymer is drained away.

In Laminated Object Manufacturing, a composite structure ofadhesive-backed paper or polymer is created by laying down thin sheetsor films with a heated roller and cutting them with a laser. The processis repeated, layer upon layer to build up a 3-D structure. Non-partareas are separated from the designed part by laser cutting and thenfurther cut into small pieces which are removed after the part isfinished. The areas removed form the channels and ports of the rocketmotor fuel grain.

In one form of 3-D Printing, a powder is laid down and a print head,similar to that of an ink-jet printer selectively sprays a fine jet ofchemical binder, which cements particles together to make a patterned,contiguous film. Another layer of powder is laid down over this and thebinder is sprayed again in another pattern, linking particles of powdertogether and to the layer below. This process is repeated until a 3-Dshape is built up. Channels left in a form where binder was not sprayedcan be cleared of unincorporated material to produce the flow channelsand ports of the rocket motor fuel grain.

In another form of 3-D Printing, a photopolymer is selectively sprayedfrom a print head and cured by radiation from a flood lamp. Successivelayers are built up to make a 3-D object. Un-printed areas in the modelare voids which form the channels and ports of the rocket motor fuelgrain.

With these methods, fuel grains can be provided with improvedperformance in compact designs.

The invention is directed to convoluted paths embedded in fuel grainsthat provide internal gas flow in a plurality of directions. Theconvoluted paths allow for a greater effective length than the physicallength of the motor, and therefore more complete oxidizer consumption.At the same time, the amount of initial surface area could be grownseveral folds by introducing truly three-dimensional surface features.The method provides greater control over the burn profile and enablescomplex three-dimensional shapes that will allow stronger fuel grainsections to be built. A small compact radial channel hybrid motor iswell suited for thrusting picosatellites, but can be scaled up to largersizes to serve more demanding thrust requirements. The hybrid rocketmotor could be built to almost any size. Those skilled in the art canmake enhancements, improvements, and modifications to the invention, andthese enhancements, improvements, and modifications may nonetheless fallwithin the spirit and scope of the following claims.

The invention claimed is:
 1. A method of making a rocket motor,comprising: selectively disposing a fuel in a thin layer, portions ofthe thin layer being selectively patterned, wherein selectivelydisposing comprises at least one of selectively disposing by laying downusing laminated manufacturing, selectively disposing by sintering usinglaser sintering, selectively disposing by extruding using fuseddeposition modeling, or selectively disposing by printing usingspraying; and repeating the selectively disposing step a plurality oftimes for disposing a plurality of the thin film layers to form a fuelgrain of the rocket motor, the patterned layers defining voids in thefuel grain.
 2. The method of claim 1, wherein the voids form structureswithin the fuel grain, the structures comprising at least one buriedchannel.
 3. The method of claim 1, wherein the voids form structureswithin the fuel grain, the structures channeling gas flow through thefuel grain when the fuel grain is burned when the rocket motor isactivated.
 4. The method of claim 1, wherein the voids form structureswithin the fuel grain, the structures channeling gas flow through thefuel grain when the fuel grain is burned when the rocket motor isactivated, the gas flowing concurrently axially and radially within thefuel grain.
 5. The method of claim 1, wherein the voids form one or morestructures within the fuel grain, the one or more structures containinga second solid fuel or liquid fuel.
 6. A method of making a solid fuelmotor, comprising: forming a three-dimensional fuel grain utilizing arapid prototyping technique comprising: selectively disposing a fuel ina thin layer, portions of the thin layer being selectively patterned;and repeating the selectively disposing step a plurality of times fordisposing a plurality of the thin film layers to form a fuel grain ofthe rocket motor, the patterned layers defining voids in the fuel grain;wherein the rapid prototyping technique comprises forming the fuel grainutilizing one of selective laser sintering, fused deposition modeling,three-dimensional printing, or laminated object manufacturing; and inthe fuel grain, at least one of the voids defining at least one channelaligned at least in part axially or radially with respect to alongitudinal axis of the fuel grain.
 7. The method of claim 6, whereindefining channels comprises defining at least one buried channel.
 8. Themethod of claim 7, wherein defining at least one buried channelcomprises defining at least one buried channel that is not exposed to aninitial gas flow within the fuel grain.
 9. The method of claim 7,wherein defining at least one buried channel comprises defining at leastone buried channel that is configured within the fuel grain to provide apredetermined burn profile of the fuel grain.
 10. The method of claim 7,wherein defining at least one buried channel comprises defining at leasttwo buried channels that differ in at least shape or size.
 11. Themethod of claim 6, wherein the fuel grain comprises a solid fuel. 12.The method of claim 6, wherein the fuel grain includes a solid oxidizer.13. A method of making a rocket motor comprising the steps of: disposinga fuel by extruding a molten material through an extrusion head in afirst patterned layer; disposing the fuel by extruding the moltenmaterial through the extrusion head in a second patterned layer againstthe first patterned layer; repeating the disposing steps a plurality oftimes to generate a plurality of patterned layers; and removing portionsof one or more of the plurality of layers to form a plurality ofpatterned layers of a fuel grain of the rocket motor, the plurality ofpatterned layers of the fuel grain defining voids in the fuel grain. 14.The method of claim 13, wherein the structures within the fuel graincomprise a second solid fuel or liquid fuel.
 15. The method of claim 13,wherein the structures channel gas flow through the fuel grain when thefuel grain is burned when the rocket motor is activated, the gas flowingconcurrently axially and radially within the fuel grain.
 16. The methodof claim 13, wherein the voids form structures within the fuel gram, thestructures comprising at least one buried channel.
 17. The method ofclaim 16, wherein the at least one buried channel comprises at least oneburied channel that is not exposed to an initial gas flow within thefuel grain.
 18. The method of claim 16, wherein the at least one buriedchannel comprises at least two buried channels that differ in at leastshape or size.
 19. The method of claim 13, further comprising laying oneor more support materials on the molten material to form the firstpatterned layer, wherein removing portions of one or more of theplurality of layers to form a plurality of patterned layers of a fuelgrain of the rocket motor comprises removing the one or more supportmaterials.